I also suggest using pytest everywhere: it works for a unit test-like structures with classes but can also be used in a simplified manner using functions. Additional features like fixtures and parametrization look very beneficial to me.
There are not enough tests, e.g., postprocessing is not tested at all. For padding, I found only this; why need GPU? Why not prepare features on CPU and compare the shapes (and the elements of the array itself)?
Re-organised unittest scripts